Method for real-time in-line testing of semiconductor wafers

ABSTRACT

An apparatus and method for the real-time, in-line testing of semiconductor wafers during the manufacturing process. In one embodiment the apparatus includes a probe assembly within a semiconductor wafer processing line. As each wafer passes adjacent the probe assembly, a source of modulated light, within the probe assembly, having a predetermined wavelength and frequency of modulation, impinges upon the wafer. A sensor in the probe assembly measures the surface photovoltage induced by the modulated light. A computer then uses the induced surface photovoltage to determine various electrical characteristics of the wafer.

RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 08/396,694,filed Mar. 1, 1995, now U.S. Pat. No. 5,661,408.

FIELD OF THE INVENTION

The invention relates to the testing of semiconductor wafers duringmanufacturing and specifically to the real-time in-line testing ofsemiconductor wafers during integrated circuit fabrication.

BACKGROUND OF THE INVENTION

There are numerous individual operations, or processing steps,performed, in a strictly followed sequence, on the silicon wafer in thecourse of manufacturing a complex integrated circuit (IC). Each suchoperation must be precisely controlled in order to assure that theentire fabrication process yields integrated circuits displaying therequired electrical characteristics.

Frequently, failure of an individual operation is detected only afterthe completion of the entire, very expensive, process of IC fabrication.Due to the very high cost of advanced IC fabrication processes, suchfailures result in the severe financial losses to the integrated circuitmanufacturer. Therefore detection of errors in the manufacturingprocess, immediately after their occurrence, could prevent theunnecessary continuation of the fabrication of devices which aredestined to malfunction, and hence, could substantially reduce thefinancial losses resulting from such errors.

Process monitoring in semiconductor device manufacturing relies upon theexamination of the changes which occur in certain physical and/orchemical properties of the silicon wafer upon which the semiconductordevices are fabricated. These changes may occur following the variousprocessing steps to which the silicon wafer is subjected and arereflected by changes in the electrical properties of the wafer.Therefore, by monitoring selected electrical properties of the siliconwafer in the course of IC fabrication, an effective control over themanufacturing process can be accomplished.

Not all of the electrical characteristics of a completed integratedcircuit can be predicted based on the measurements performed on apartially processed wafer. Most of the characteristics however, can bepredicted directly or indirectly based on the investigation of thecondition of the surface of the silicon wafer (substrate) in the courseof IC manufacture. The condition of the silicon surface is verysensitive to the outcome of the individual processing steps which areapplied during IC manufacturing, and hence, the measurement of theelectrical properties of the substrate surface can be an effective toolby which the monitoring of the outcome of the individual processingsteps can be accomplished.

The determination of the electrical characteristics of the wafer surfacetypically requires physical contact with the wafer surface, or theplacement of a contactless probe over a stationary wafer. In the lattercase an optical signal or a high electric field is used to disturbequilibrium distribution of the electrons in the surface andnear-surface region of semiconductor. Typically, the degree of departurefrom equilibrium is driven by variations of one or more electricalcharacteristics of the surface region, the near-surface region, and thebulk of the semiconductor. To obtain a more complete picture of theentire surface of the wafer, several measurements at various points onthe surface can be made. Such a procedure, known as "mapping", moves themeasuring probe with respect to the measured material (or vice versa)over the surface of specimen, stopping at a number of locations andperforming a measurement at each location before moving to the nextlocation. The substrate, in this procedure, does not remain in thecontinuous motion, so consequently the applicability of such a methodfor use in real-time in-line process monitoring is limited.

SUMMARY OF THE INVENTION

The invention relates to an apparatus and method for the real-time,in-line monitoring of semiconductor wafer processing. In one embodimentthe apparatus includes a probe assembly located within a semiconductorwafer processing line. As each wafer is carried beneath or above theprobe assembly by conveyor belt, robotic arm, wafer chuck, or othersimilar devices a source of modulated light, such as an LED, within theprobe assembly, generates light having a predetermined wavelength andfrequency of modulation which then impinges upon the wafer. A sensor inthe probe assembly measures the surface photovoltage induced by themodulated light. The signal from the sensor is sent to a computer whichthen uses the induced surface photovoltage to determine variouselectrical characteristics of the wafer, such as surface charge andsurface doping concentration, among others.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention is pointed out with particularity in the appended claims.The above and further advantages of this invention may be betterunderstood by referring to the following description taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram of an embodiment of an apparatus for thereal-time, in-line, electrical characterization of a semiconductorduring manufacturing;

FIG. 2 is a perspective view of an embodiment of the probe assembly ofthe apparatus of FIG. 1 in position above a wafer transfer system;

FIG. 3 is a top perspective cutaway view of the probe assembly of FIG.2;

FIG. 4 is a bottom perspective view of an embodiment of the senor plateof the probe assembly of FIG. 3;

FIG. 5 is a schematic diagram of an embodiment of an electrical circuitfor measuring the surface photovoltage using front wafer surfacecoupling;

FIG. 6a depicts a block diagram of a corona control circuit used tocharge a wafer so as to generate an inversion layer at the wafersurface; FIG. 6b depicts a block diagram of the corona control circuitof FIG. 6a used to discharge a wafer;

FIG. 7 is a bottom perspective cutaway view of an embodiment of thecoated sensor plate of FIG. 4 with a polyimide coating, used with sensorcharging and high voltage biasing;

FIG. 8 is a schematic diagram of an embodiment of a preamplifier circuitused for the high voltage biasing of the wafer using the sensorelectrodes; and

FIG. 9 is a graph of front and back surface charge measurements of asilicon wafer undergoing cleaning.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In one embodiment, the apparatus to perform various electricalcharacterizations makes use of the method for measuring thephoto-induced voltage at the surface of semiconductor materials, termedthe surface photovoltage (SPV), disclosed in the U.S. Pat. No.4,544,887. In this method, a beam of light is directed at a region ofthe surface of a specimen of semiconductor material and thephoto-induced change in electrical potential at the surface is measured.The wavelength of the illuminating light beam is selected to be shorterthan the wavelength of light corresponding to the energy gap of thesemiconductor material undergoing testing. The intensity of the lightbeam is modulated, with both the intensity of the light and thefrequency of modulation being selected such that the resulting ACcomponent of the induced photovoltage is directly proportional to theintensity of light and inversely proportional to the frequency ofmodulation.

When measured under these conditions, the AC component of the surfacephotovoltage (SPV), designated δV_(s), is proportional to the reciprocalof the semiconductor space-charge capacitance, C_(sc). When the surfaceof the specimen is illuminated uniformly, the relationship between thesurface photovoltage (SPV) and the space-charge charge capacitance isgiven, at sufficiently high frequencies of light modulation, by therelation: ##EQU1## where Φ is the incident photon flux, R is thereflection coefficient of the semiconductor specimen, f is the frequencyat which the light is modulated, and q is the elementary charge. Theconstant K is equal to 4 for a square wave modulation of the lightintensity and is equal to 2π for sinusoidal modulation.

In the above referenced patent, only a uniform configuration isconsidered in which the area of the sensor is at least the same size asthe semiconductor wafer and the entire area of the specimen is uniformlyilluminated. When only a portion of the semiconductor specimen surfaceis coupled to the sensor, that is, when the sensor is smaller than thewafer, and when the semiconductor surface uniformly illuminated in thatarea is coupled to the sensor, the surface photovoltage, δV_(s), may bedetermined from the measured signal, δV_(m), according to therelationships:

    Re(δV.sub.s)=Re(δV.sub.m)·(1+C.sub.L /C.sub.p)+Im(δV.sub.m)·(ω·C.sub.p ·R.sub.L).sup.-1

    Im(δV.sub.s)=Im(δV.sub.m)·(1+C.sub.L /C.sub.p)-Re(δV.sub.m)·(ω·C.sub.p ·R.sub.L).sup.-1

where Re(δV_(s)) and Im(δV) are the real and imaginary components of thevoltage, ω is an angular frequency of light modulation, C_(p) is thecapacitance between sensor and the wafer, and C_(L) and R_(L) are theinput capacitance and resistance, respectively, of the electronicdetection system.

From the sign of the imaginary component, the conductivity type may bedetermined. If the measurement is calibrated for a p-type material, thenthe sign of the imaginary component will change if the material isn-type.

Using above relationships, the depletion layer width, W_(d), is given byequation: ##EQU2## where φ(1-R) is the intensity of light absorbed inthe semiconductor, q is the elementary charge, and ε_(s) is thesemiconductor permittivity.

In addition to the space-charge capacitance, C_(sc), the measurement ofthe surface photovoltage can be used to determine the surface chargedensity, Q_(ss), the doping concentration, N_(sc), and the surfacerecombination lifetime, τ, using the following relationships. The spacecharge capacitance, C_(sc), is proportional to the reciprocal of thesemiconductor depletion layer width, W_(d), according to therelationship: ##EQU3## where ε_(s) is the semiconductor permittivity.The density of space charge, Q_(sc), is in turn described by equation:

    Q.sub.sc =q N.sub.sc W.sub.d

where q is an elementary charge and the net doping concentration in thespace-charge region, N_(sc), is positive in an n-type material andnegative in a p-type material. In addition, since the surface chargedensity, Q_(ss), is given by the expression:

    Q.sub.sc =-Q.sub.ss

the surface charge density is easily determined from the space chargedensity.

Further, if an inversion layer can be created at the wafer surface, thedepletion layer width, W_(d), under inversion conditions is related tothe net doping concentration, N_(sc), according to the relationship:##EQU4## where kT. is the thermal energy and n_(i) is the intrinsicconcentration of free carriers in the semiconductor. Several methods offorming such an inversion layer at the semiconductor surface aredisclosed below.

Finally, the surface recombination rate may also be determined from theSPV. The recombination lifetime of the minority carriers at the surface,τ, is given by the expression: ##EQU5##

In brief overview, and referring to FIG. 1 an embodiment of such anapparatus 10 for the real-time, in-line, electrical characterization ofa semiconductor during manufacturing using induced surface photovoltageincludes a sensor head assembly 14, supporting electronics 18, and awafer conveying device 22. In operation, the wafer conveying device 22,such as a conveyor belt, a robotic arm, a wafer chuck or similar device,moves wafers 28, 28' through the manufacturing process and, in oneembodiment, beneath the sensor head assembly 14.

Referring to FIG. 2, the sensor head assembly 14 includes a probe head32 mounted in a bracket 36 on a motorized stage 40. The motorized stage40 moves the probe head 32 in a vertical direction (arrow z) to adjustvertical position of the probe head 32 with respect to the wafer 28 towithin a 0.2 μm accuracy. The mechanical stage 40 is attached to a probearm 44.

The longitudinal axis L-L' of the probe head 32 is adjusted to beperpendicular to the plane of the wafer 28, by adjusting the tilt of theprobe arm 44, either manually (using set screws 46) or mechanically(using for example piezoelectric actuators 48). The vertical position ofthe probe head 32 with respect to the wafer 28 is controlled by feedbacksignal from capacitive-position sensing electrodes described in detailbelow.

Briefly, three capacitive-position sensing electrodes are located on theperiphery of the sensor. To measure capacitance between each of theseelectrodes and the wafer, a 70 kHz 1V signal is applied through arespective 10 kohm resistor connected to each of these electrodes. TheAC current flowing through these resistors in measured using apreamplifier and a lock-in amplifier. The lock-in signal is furtherprocessed by a computer and supplied to the motion control board that,in turn, positions the probe at a predetermined distance from the wafersurface using vertical (z-axis) motorized stage.

Referring to FIG. 3, the probe head 32 includes a sensor mount assembly50 which provides support for a sensor 54 that is connected to apreamplifier board 58 by a plurality of flexible connectors 60. Lightemitted by a light emitting diode (LED) 64 is collimated by lens 68prior to passing through a beam splitter 72.

LED 64 is mounted on a LED driver board 74 which controls the intensityof the LED 64, in response to a signal from a reference photodiode 78,(through a preamplifier 79) at an intensity level determined by thecomputer 160. Light from the LED 64 reaches the reference/photodiode 78by being partially reflected by the beam splitter 72. The light whichpasses through the beam splitter 72 passes through openings 80, 82 inthe circuit board 86 and the preamplifier board 58, respectively, priorto passing through the sensor mount assembly 50 and impinging on thewafer 28 undergoing testing.

Light reflected by the wafer 28 passes back along the light path justdescribed before being reflected by the beam splitter 72 to a measuringphotodiode 92. The light reflected by the wafer 28, Φ_(R), is used todetect edge of the wafer passing beneath the probe head 32 and triggermeasurements. The reflected light is also used to measure light absorbedin the wafer 28 according to the relationship:

    Φ=Φ.sub.0 -Φ.sub.R

where Φ₀ is the incident light which can be determined by measuring thelight reflected from an aluminum mirror replacing the wafer 28. In thisway, the reflection coefficient of the wafer 28 can be determined.Although the above embodiment describes the splitting of light by a beamsplitter, other embodiments are possible in which light is split usingoptical fibers.

Referring again to FIG. 1, the LED 64 is controlled by signals from, andthe probe head 32 returns signals to, supporting electronics 18. Thesupporting electronics 18 include an oscillator 100 which supplies a 40kHz modulation control signal 104 that is used as a reference signal byan LED control 62 to control an LED driver 63 which powers the LED 64.Oscillator 100 also provides a reference signal 108 to a lock-inamplifier 112. The output signals 116 from the surface photovoltagesensor and the measurement photodiode 92 (through a preamplifier 93) ofthe probe head 32 are input signals to multiplexer 120 that alternatelyconnects each signal to the input of the lock-in amplifier 112. Thelock-in amplifier 112 demodulates the input signal and supplies thedemodulated signal to another multiplexer 150. Multiplexer 150 switchesbetween the two input signals from lock-in amplifiers 112 and 140connecting them to a data acquisition (DAQ) board 156 that in turndigitizes the input signals making them available for further processingin the computer 160. In an alternate embodiment, multiplexer 150 is partof the data acquisition board 156.

FIG. 4 is a bottom perspective view showing the sensor plate of thesensor head 32. A plurality of electrodes are formed on a rigid andinsulating substrate 200. In one embodiment, a 10 mm diameter fusedquartz disc is used. A central surface photovoltage electrode 204detects the signal from the wafer 28. The central surface photovoltageelectrode 204 is partially transmissive, thereby permitting the lightfrom the LED 64 to reach the wafer 28. Three other electrodes 208located on the periphery of the substrate are used both for sensing theposition of the sensor head 32 above the wafer 28 and for measuring theparallelism of the sensor with respect to the surface of the wafer 28.All electrodes 204, 208 are formed by the deposition of anindium-tin-oxide film through a shadow mask.

Similarly, a plurality of electrodes 212, for connecting the sensorswith the preamplifier circuit board 58 through the flexible connectors60, are formed on the surface of the substrate 200 which is opposite theelectrodes 204, 208. Thin conductive electrodes 218, on the side wallsof the substrate 200, which connect the electrodes 204, 208 on the firstsurface with their respective electrodes 212 on the second surface, arealso deposited using a shadow mask. This deposition avoids the use ofvias through the substrate and thereby retains the flatness of thesensor to better than 0.2 μm. Both front 204, 208 and side electrodes218, may be protected with a thin insulating coating, such as polyimide,formed by spinning so as to maintain the flatness of the sensor.

The electrodes 208 are used for capacitively sensing the position of thesensor above the wafer 28. Referring again to

FIG. 1, a 70 kHz input signal 124 for measuring the distance from awafer 28 is supplied by an oscillator 128 to the position electrodes208. The same signal is also supplied as a reference signal 132 for alock-in amplifier 140. A position signal 146 from each of the threeposition sensing electrodes 208 is supplied as the input signal to amultiplexer 148 through a preamplifier 149. The multiplexer 148 in turn,switching between each of these signals, connects each alternately to alock-in amplifier 140. The demodulated output signals from the lock-inamplifiers 112 and 140 are input signals to a multiplexer 150 whichconnects each signal alternately to a data acquisition board 156 locatedin a computer 160, including a CPU 164. Again, in an alternativeembodiment, multiplexer 150 is part of the data acquisition board 156.

The position signal 146 is compared by the CPU 164 with the referencevalue corresponding to a desired distance (established by calibrationand stored in the computer) between the sensor 54 and 18 the wafer 28.The difference between these two values, corresponds to the deviation ofthe sensor-wafer distance from the desired value, is supplied to amotion control board 170 that positions the probe head 32 at apredetermined distance from the wafer 28 using the motorized stage 40.

In operation, when an edge of the continuously moving wafer 28 crossesthe beam of the intensity modulated light from LED 64, the intensity ofthe reflected light increases, thereby increasing the signal from thephotodiode 92. This measurement of the reflected light is repeated andthe new value compared with the previous value. The light intensitymeasurements are repeated until the difference between sequential valuesdecreases to below 5% indicating that the entire light beam is withinthe flat portion of the wafer.

This decrease in deviation triggers acquisition of the SPV signal by thesurface photovoltage electrode 204, followed by acquisition of thecapacitance signals by the position electrodes 208. If capacitancesignals from different electrodes (208) differ by more than 5%, the SPVsignal is stored but not recalculated. The sequence of all measurementsis then repeated until capacitances from different position electrodes(208) fall within 5% limit indicating that the electrodes are not nearthe edge of the wafer 28. The average of the capacitances from the threepositioning electrodes 208 at this point is used to recalculate allprevious values of the SPV signal.

The SPV measurement cycle is repeated, sequentially measuring lightintensity, SPV signal and capacitance of positioning electrodes, untilcapacitances from the three positioning electrodes (208) differ by morethan 5%, indicating the approach of the opposite edge of the wafer 28.After reaching this point of the wafer 28, the SPV measurements are madeusing the previously measured values of capacitance. The measurements ofeach value (reflected light, SPV signal, capacitance), in each cycle,are repeated for 10 msec and averaged by CPU 164.

The wafer 28, in one embodiment, is placed on the grounded chuck(conveyor belt, robotic arm, or other similar device) 178, coated withan insulating material, that is used to carry the wafer 28 beneath,above, or otherwise, such that the surface of the sensor of the probehead 32 and the surface of the wafer are parallel. Alternatively, theconveying device may be biased by a DC voltage. In one embodiment the DCbias voltage is selected to be between -1000 and 1000 volts. AlthoughFIG. 1 illustrates the use of a grounded, insulated chuck 22 to move thewafer 28 beneath the probe assembly 14, it is possible to provide allthe necessary measurements without grounding the chuck using only theelectrodes provided by the sensor 54. Referring to FIG. 5, the SPVsignal is, as described previously, received by the central surfacephotovoltage electrode 204 which is connected to the input terminal ofan operational amplifier 250 located on the preamplifier circuit board58. The other input terminal of the operational amplifier 250 isconnected to ground and to the output terminal of the operationalamplifier 250 through one or more resistors. What was previously a backcapacitive contact, supplied by the chuck, is now provided by the threepositioning,electrodes 208 located on the periphery of the sensor andwhich, during the SPV measurements, are connected to the ground 252rather than to the input terminal of the capacitance (current measuring)preamplifier located on the preamplifier circuit board 58.

To measure capacitance, the electrodes 208 are alternatively switchedbetween the ground 252 and input of the capacitance preamplifier locatedon preamplifier circuit board 58. This arrangement makes possiblenon-contact measurements with any type of wafer support. Thus, the wafersupport does not need to be connected to ground and could be made ofinsulating material.

As discussed above, measurements of the surface doping concentrationrequire the formation of an inversion layer at the wafer surface. In oneembodiment this is accomplished by charging the wafer 28 using a coronagenerator and subsequently performing surface photovoltage measurementon the wafer 28. Specifically, the wafer 28 is first charged toinversion with a corona generator. N-type wafers require a negativesurface charge and p-type wafers require a positive surface charge. Inone embodiment, the corona generator includes a single metal tip, forexample tungsten, located 5 mm above the wafer 28 and biased to 3.5 kVfor 2 to 3 sec. After charging, the wafer 28 is moved beneath the probeassembly 14 and the measurements performed. After the measurement, thewafer 28 is either moved beneath a neutral charge corona generator orreturned to the original corona generator operated in a neutraldischarge mode in order to discharge the wafer.

The simple corona generator with the metal tip or wire does not allowfor the controlled charging of the wafer surface. The control ofcharging is important because while there is a minimum charge requiredto induce an inversion layer at the wafer 28 surface, overcharging maydamage the wafer surface, and even cause electrical breakdown of theinsulating coating formed on the wafer surface. To avoid overchargingthe wafer 28, a closed loop controlled corona charging arrangement,disclosed in FIGS. 6a and 6b, controls the charge deposited on thesurface of the wafer and thereby prevents surface damage.

Referring to FIG. 6a, the wafer 28 on the grounded, insulated chuck 22is moved beneath an ionized air source 260 located about 10 mm above thewafer 28. A mesh, stainless steel, reference electrode 264 is placed ina distance of about 0.5 mm to 1 mm from the wafer 28. The differencebetween the potential on the reference electrode 264, V_(el), and a userdefined and computer generated reference voltage, V_(ref), 268, termedthe differential potential, V_(diff), is amplified and its polarity isreversed within the corona control module 270. This voltage, V_(corr),is applied to the ionized air source 260. Thus, the polarity of thepotential applied to the ionized air source 260, V_(corr), by the coronacontrol module 270 is opposite to the polarity of differential voltageand is given by the expression:

    V.sub.corr =V.sub.ref -V.sub.el

Control of the corona charging during the charging process allows notonly for real-time control but allows also simpler electronic circuitryto be used. The presence of the ions between ionized air source 260,reference electrode 264, and the wafer 28 lowers the equivalentimpedances in the circuity and permits amplifiers to be used (in thecontrol module 270) which have an input impedance of 10⁹ -10¹⁰ ohms.This input impedance is several orders of magnitude lower than in theamplifiers utilized in previous approaches (typically 10¹³ -10¹⁵ ohms)when a potential of the wafer surface is measured not during chargingbut after the turning off of the corona.

Referring to FIG. 6b, the wafer 28 may be discharged by setting thereference voltage 268 to zero, i.e., connecting it to ground.Alternatively, if separate corona units are used for charging anddischarging of the wafers, the discharging corona reference voltage canbe permanently attached to the ground.

Referring to FIG. 7, an alternative approach to inducing a surfaceinversion layer is to bias the sensor with a high voltage. Such anapproach requires formation of the insulating film 230 such as polyimideover the central electrode 204 and positioning electrodes 208 of thesensor. FIG. 8 depicts this alternative approach to inducing aninversion layer at the surface of the wafer 28 by voltage biasing. FIG.8 shows a schematic diagram of an electronic circuit that includes apreamplifier for measuring AC surface photovoltage and a connection to abiasing high voltage source used with the sensor having a polyimidecoating 230 as just described. The insulating coating 230 of the sensor54 allows the application of a high enough voltage (500-1000 V) toinduce a surface inversion layer in typical wafers used inmanufacturing. The arrangement in which a rigid sensor electrode 204 isseparated by an air gap from the semiconductor surface requires highdegree of flatness of the electrode surface. When such a high DC voltageis used, any edges or surface roughness will increase the localelectrical field and enhance ionization of the air resulting inelectrical breakdown. Therefore electrical connections between theelectrode and the detection electronics are constructed so as to have aminimal effect on the surface flatness. Thus, the use of the sideconnections 218 eliminates the need to form via holes in the sensor andmaintains the high flatness of the sensor. The current in the spacecharge region of the wafer 28 (indicated in phantom) which is generatedby the illumination of the wafer 28 by the LED 64 is depicted as anequivalent current source, J_(h). An equivalent resistor, R_(R), whichrepresents the carrier recombination at the surface of the wafer 28 andan equivalent capacitor, C_(SC), which represents the space chargecapacitance are also depicted. C_(G) represents capacitance between thewafer 28 and the chuck 22, while C_(p) represents capacitance betweenthe sensor electrode 204 and the wafer 28. A computer controllable highvoltage 300 is applied through a 10 Mohm resistor, R_(HV), to the sensorelectrode 204. The sensor electrode 204 is also connected to the inputof the operational amplifier 250 (described previously) through a highvoltage capacitor, C_(HV). The capacitance, C_(OA), (also shown inphantom) represents input capacitance of the operational amplifier 250.C_(HV) is selected to be about 10 times larger than C_(OA) so that C_(L)used in calculating Im(δV_(s)) and Re(δV_(s)) is close to C_(OA).Similarly R_(L) used in calculating Im(δV_(s)) and Re(δV_(s)) is closeto R_(HV).

In addition to the methods just described to form an inversion layer, aninversion layer at the surface of the wafer 28 can be also formed usinga chemical treatment. This approach is especially useful for p-typesilicon wafers. Since HF introduces positive surface charge, HFtreatment will produce a negative inversion layer at the surface ofp-type silicon wafers. In one embodiment, the silicon wafer to be testedis subjected to a mixture of hydrofluoric acid and water (1:100 HF:H₂ O)in a liquid or vapor form. The wafer is then placed beneath the probeassembly 14. In number of processes, HF treatment is already part of theproduction sequence so that probe assembly 14 needs only to be placedafter HF processing location.

It should be noted that the formation of an inversion layer is useful inmeasuring conductivity type.

Since, in some cases, incoming wafers show acceptor neutralization dueto the presence of hydrogen or copper, in order to restore the dopingconcentration at the surface, the measured wafer is subjected to a highintensity illumination (e.g., using a 250 W halogen light source) aftera SPV measurement is made.

Additionally, the present apparatus is particularly adaptable for use ina sealed chamber environment, such as a reduced pressure chamber, achamber for chemically reactive gasses or a chamber for an inertenvironment. The entire probe assembly 14 may be positioned within thesealed chamber, with the connections to the electronics passing throughthe walls of the sealed chamber through pressure fittings.Alternatively, the probe assembly may be mounted in a wall of a sealedchamber such that the sensor is positioned within the chamber but theremainder of the probe assembly is positioned outside of the sealedchamber.

The approach to process monitoring methodology using an AC-SPV methodemphasizes determination of variations of the measured parameters fromwafer to wafer rather than value of the specific parameter itself.Typically, measurements of the electrical parameters of the back surfaceof the wafer are not possible without altering the front surface, whichhas to be contacted in order to complete a measuring circuit. Hence,measurements performed on the back surface of the wafer are nottypically used in process monitoring. The non-contact AC-SPVmeasurements allows process monitoring by measurement of the surfacecharacteristics on the back surface of the wafer as well as the frontsurface. As described before, the probe head can be installed underneaththe wafer, above the wafer, or otherwise, such that the sensor surfaceis parallel to the wafer back surface, depending on how the waferconveying system conveys the wafer to the probe head. In addition, twoprobe heads can be used, one on each side of the wafer for simultaneouscharacterization of the front and back side of the wafer. As anillustration of such approach comparison of measurements of the surfacecharge on the front surface featuring mirror-like finish is shown inFIG. 9. The measurements were performed on the two halves of the same100 mm, p-type, (100) silicon wafers that were simultaneously subjectedto the wet cleaning treatments. At various stages of the cleaningprocess, the surface charge was measured on the front (polished) surfaceof one half, and on the back (unpolished) surface of the other half. Theresults shown in FIG. 9 indicate identical behavior of surface charge onthe front and back surfaces.

Having shown the preferred embodiment, those skilled in the art willrealize many variations are possible which will still be within thescope and spirit of the claimed invention. Therefore, it is theintention to limit the invention only as indicated by the scope of thefollowing claims.

We claim:
 1. A method of restoring a doping concentration at the surfaceof a wafer comprising the steps of:providing a wafer; and exposing saidwafer to broad band illumination.
 2. The method of claim 1 wherein saidstep of exposing said wafer to illumination comprises illumination witha 250 W halogen light source.